Showing papers on "Spatial filter published in 1980"

Digital Image Enhancement and Noise Filtering by Use of Local Statistics

Abstract: Computational techniques involving contrast enhancement and noise filtering on two-dimensional image arrays are developed based on their local mean and variance. These algorithms are nonrecursive and do not require the use of any kind of transform. They share the same characteristics in that each pixel is processed independently. Consequently, this approach has an obvious advantage when used in real-time digital image processing applications and where a parallel processor can be used. For both the additive and multiplicative cases, the a priori mean and variance of each pixel is derived from its local mean and variance. Then, the minimum mean-square error estimator in its simplest form is applied to obtain the noise filtering algorithms. For multiplicative noise a statistical optimal linear approximation is made. Experimental results show that such an assumption yields a very effective filtering algorithm. Examples on images containing 256 × 256 pixels are given. Results show that in most cases the techniques developed in this paper are readily adaptable to real-time image processing.

Real‐time image processing via four‐wave mixing in a photorefractive medium

Abstract: We report the application of four‐wave mixing to real‐time image processing. We constructed a nonlinear optical processor using a photorefractive medium Bi12SiO20 and demonstrated that it is capable of convolving and correlating objects with spatial information.

Frequency dependence of modal noise in multimode optical fibers

Abstract: Under certain conditions a changing speckle pattern exists at the output plane of a multimode fiber, resulting in modal noise which can degrade the error performance of a fiber data link. Fiber motion is the usual cause of such speckle change, and Daino et al. have studied the first-order statistics of such modal noise, assuming a single frequency source. But source frequency variation can also cause modal noise; and source frequency diversity has been shown effective in its reduction. In this paper we use a speckle theory approach to study the frequency dependence of modal noise. We have measured and analyzed the correlation of two speckle patterns as a function of source frequency difference, and the speckle spatial frequency distribution as a function of fiber parameters. We have also measured the speckle contrast as a function of fiber length for several sources and fiber types. Such information permits the prediction of the modal noise statistics, from which corresponding changes in error rates can be derived.

Apparatus for inspecting defects in a periodic pattern

Abstract: An apparatus for inspecting a defect in a periodic pattern on an object is provided with a laser device for projecting a laser beam toward the object. A mechanism further included in the defect inspecting apparatus rotates the object in a plane orthogonal to an optical path of the laser beam while moving in the same plane. A spatial band-pass filter is located at the backward focal plane of a lens for Fourier-transforming the laser beam including the information of the periodic pattern and a defect, the laser beam coming from the object. The band-pass filter has a spot-like area for blocking the zeroth order diffraction light transmitted through the periodic pattern a peripheral light blocking area for blocking the first and higher order diffracted light beams, and a ring-like light transmission area permitting the light beam component including the information of a defect to pass therethrough, the light transmission area being located between the spot-like area and the peripheral area. A photo-electric converter for picking up the light beam component including the defect information transmitted through the filter is located on an image forming plane where an image of the object is formed by a lens.

Radial birefringent element and its application to laser resonator design.

Abstract: We have invented a quasi-Gaussian profile-transmittance filter based on radially varying the phase retardation in a birefringent element. The radial birefringent element has been applied to resonator design and has demonstrated its usefulness in generating an improved resonator spatial-mode profile.

High-power phosphate-glass laser system: design and performance characteristics

Abstract: A one-beam prototype of a large twenty-four-beam phosphate-glass laser system has been built and tested. Basic design characteristics include Nd-doped phosphate glass, rod amplifiers up to 90-mm diam, the propagation of a circularly polarized beam, extensive spatial filtering and imaging, and the use of large-aperture Pockels cells. The prototype system has demonstrated focusable power in excess of 750 GW/beam at 50-psec full width at half-maximum (FWHM) pulse duration and has delivered 165 J of focusable energy in a 500-psec (FWHM) pulse. Maximum beam brightness has been measured to be 4 × 1019 W/cm2 · sr at a firing rate of 2 shots/hr. Peak-to-background energy contrast of 108 has been achieved with this system. A detailed description of the system design and performance is presented.

Achromatic interferometers for white light optical processing and holography.

Abstract: Achromatic interferometers are developed that perform optical processing operations and record both the phase and amplitude of the output by means of a coherent reference beam. The interferometers simultaneously carry out the data processing tasks and form fringes with white light extended sources.

Measurement of refractive-index profiles in optical-fibre preforms by spatial-filtering technique

Abstract: A nondestructive technique based on spatial filtering is presented for measuring the refractive-index profile in an optical-fibre preform. The method is simple, accurate and has sufficient resolution to enable detailed profiles of single-mode preforms to be obtained.

Generalized matched filtering.

Abstract: Matched filtering is the key concept in coherent optical pattern recognition. Unfortunately, matched filters are more sensitive to some things (nonconsequential variations in size, rotation angle, etc.) and less sensitive to other things (differences between object classes) than we might desire. The generalized matched filter is a filter tailored to the specific requirements of the task. It contains the matched filter as a special case.

Optical noise and temporal coherence

Abstract: Previous articles have been devoted to the study of optical noise as a function of spatial coherence. The present one completes this study by considering temporal coherence. Noise arising from defects in the pupil plane and affecting the high spatial frequencies of an image is notably reduced by white-light illumination. Temporal coherence has little effect on noise arising from defects in the object plane. However, impulse noise due to small isolated defects is reduced in size. Physical arguments are presented to explain these phenomena and a mathematical study of partially coherent imaging in the presence of random defects is given.

Rotational matched spatial filter for biological pattern recognition.

Abstract: Randomly oriented microbiological patterns are recognized by rotating a matched spatial filter with an optical wedge prism. The filter is made from a composite input pattern with various sized objects to cover wider ranges of size variation in a species to be identified.

Tunable spatial filtering with a Fabry-Perot etalon.

Abstract: A Fabry-Perot etalon is used in the image plane as a means of selecting arbitrary directions of propagation in the angular spectrum of an object. This provides a simple widely tunable spatial filter that could be useful in a number of image processing and pattern recognition applications.

Real-time white light spatial frequency and density pseudocolor encoder.

Abstract: A real-time white light pseudocolor encoding technique for spatial frequency and density encodings is presented. In spatial frequency color coding, the encoding is accomplished by spatial filtering of the color signal spectra, while in density pseudocoloring, the encoding consists of contrast reversal of a color object image. The technique is simple, versatile, and economical to operate, which may offer some practical applications. Because the encoding colors are primarily derived from a white light source, the annoying coherent artifact noise can be substantially reduced. Since the encoding is obtained with a broad spatial band of the signal spectra, this technique offers no apparent resolution loss. We stress that this real-time white light pseudocolor encoding technique may offer several major advantages that previous techniques have offered. Experimental demonstrations of this pseudocolor encoding technique are also provided.

Method and apparatus for optical computing and logic processing by mapping of input optical intensity into position of an optical image

Abstract: Arrangements and methods are described employing input optical intensity to output spatial position mapping, for performing optical computing or logic processing functions. An electro-optical device which receives discrete input optical object beams of different intensity levels and a separate readout optical beam, diffracts portions of the readout optical beam to unique spatial positions in two dimensional space, each portion as a function of the intensity of individual discrete input beams or of combinations of the intensities of the input beams. A spatial filter having discrete apertures at selected ones of the unique spatial positions transmits the selected diffracted optical outputs for implementing logic or computing functions.

Optical logic with variable-grating-mode liquid-crystal devices.

Abstract: A liquid-crystal device that performs a two-dimensional intensity-to-spatial-frequency conversion has been investigated for use as an optical transducer. When such a device is used as the input transducer in an optical filtering arrangement, image-intensity levels can be easily manipulated by using appropriate Fourier plane filters. The variable-grating-mode liquid-crystal device is well adapted to performing binary logic operations on images. Results demonstrating the implementation of common logic operations are presented.

Generation of arbitrarily shaped optical pulses in the subnanosecond to picosecond region using a fast electrooptic deflector

Abstract: New types of optical-pulse shaper and compressor in the subnanosecond to picosecond region are proposed, in which a fast electrooptic deflector is used together with Fourier optical elements (e.g., spatial fdter, lens, slit, grating, etc.). In the preliminary experiment using a LiTaO 3 electrooptic-deflector device and a CW 633 nm HeNe laser, various types of shaped pulse, for example, Gaussian, squared, triangle, and double-peak pulses, were obtained with 94 ps minimum duration.

Photoelectric device for the recording of diameter changes of opaque and transparent blood vessels in vitro.

Abstract: A new photoelectric device for measuring blood vessel diameters is described. The principle of this device consists in locating the vessel within a beam of parallel light at right angles to the beam direction, and eliminating all light striking the vessel. Thus only the light passing by the side of the vessel determines the signal strength of a photocell. The elimination of the light by the vessel due to reflection, refraction, diffraction, or scattering, is achieved with the aid of a lens and pinhole representing a spatial filter. This arrangement is effective irrespective of whether the vessel is opaque or transparent. The resolving power of the device in measuring changes of outside diameter is better than 0.5 μm for vessels up to 3 mm in diameter. The upper frequency limit is 300 Hz (−3 dB). The application of the method is demonstrated by two examples of measurements obtained on a small muscular artery.

Small-scale self-focusing of laser radiation in amplifier systems

Abstract: A matrix method is developed for the calculation of the evolution of small-scale self-focusing as applied to laser amplifier systems, i.e., active elements with a nonlinear refractive index, separated by air gaps or lens systems. Calculations made for collimated and diverging laser beams indicate that in amplifier systems it is preferable to use spatial filters with beam expansion. These calculations can be used to optimize their parameters.

Phase-conjugate interferometer

Abstract: A speckle interferometer including a beam splitter, a mirror in the object beam arm, a phase-conjugate mirror in the reference beam arm, a converging lens and a photographic film. Laser light scattered retro-reflectively from a rough surface under investigation and passed through an imaging lens illuminates the interferometer. Fringes occur upon sandwiching a pair of exposures of the interference pattern made before and after deformation of the rough surface. The relative magnitude of the displacements from the original position at different points of the surface can be determined from the position of the fringes.

Coherent optical feedback for the analog solution of partial differential equations

Abstract: A confocal Fabry-Perot interferometer is used to produce feedback within an optical processor. The resulting confocal feedback system (CFS) has a complex-valued feedback transfer function and performs analog solution of partial differential equations (PDE’s). The CFS is interfaced to a microcomputer via a laser scanner and video electronics to create a hybrid processor, thus taking a major step toward the practical use of the CFS for the high-speed solution of PDE’s. Solutions to the three types of second-order linear PDE’s in two dimensions—elliptic, hyperbolic, and parabolic—were obtained experimentally and analyzed.

Beyond matched filtering

Abstract: Matched filtering is often too sensitive to within-class variations (rotation, translation, magnification, etc.) and too insensitive to between-class variations. Improvements can be sought by changing the input, changing the filter, or changing the way the data is handled. We review both systematic and ad hoc methods to accomplish those improvements. The resulting improvements make coherent optical pattern recognition far more versatile in 1980 than it was when it began in 1964.

White-light pseudocolor density encoding through contrast reversal.

Abstract: A simple technique of pseudocolor density encoding of photographic image by white-light processing is described. Spatial encodings are made through positive and negative photographic-image transparencies, and the pseudocoloring is obtained by color filtering of the smeared Fourier spectra. The technique is simple, versatile, and economical to operate. Since coherent sources are not used, annoying coherent artifact noise can be eliminated. Spatial color encoding uses the entire Fourier spectrum; therefore, this technique offers no apparent resolution loss. Simple experimental demonstrations of this technique are provided.

Maximum entropy spectral analysis in antenna spatial filtering

Abstract: An eight-element linear receiving array working in G band is simulated The azimuthal response is formed using a maximum entropy algorithm This gives the antenna a remarkable ability to detect low-level signals in the presence of high-level signals The sidelobes encountered when using Fourier analysis do not seem to exist The resolution of two equal targets can be made to depend on the signal-to-noise ratio only, by averaging the prediction coefficients from many data sets containing random phase difference information from the two targets

Multiple wavelength instrument for measurement of particle size distributions

Abstract: A method for measurement of the size distribution of particles suspended in a gas or in a liquid The particle suspension is illuminated by a collimated beam of colored light, which is produced by passing a collimated beam of substantially white light through a spectral filter which has spatial sections of narrow spectral passband in the blue, green and red Part of the light scattered by the particles is collected by a lens and is passed through a spatial filter placed in the focal plane of the lens The light transmitted by the filter is measured by a photodetector The photodetector output is measured as different spatial filters are switched in place A computer, microprocessor, or analog device acts on the measured values and produces the particle size distribution as an output The data reduction algorithm consists of a linear transformation of the measured data vector, followed by the construction of a linear combination of basis functions for the size distribution The spatial filters consist of color transparencies with non-uniform spatial transmittance functions in the blue, green, and red The average scattering angle is not restricted, but for broad polydisperse particle distributions near back scatter is preferred, in order to avoid an ill-conditioned data reduction problem One of the spatial filters has uniform transmittance for the purpose of back ground light subtraction, and to provide a bias in order to allow effectively indefinite filter transmittance functions

PLZT spatial light modulator for a 1-D hologram memory

Abstract: A 100-bit slitlike aperture array spatial light modulator has been developed using 88/65/35 PLZT for a high-bit-density 1-D hologram memory The most desirable characteristics, such as distortion-free diffraction-limited Fourier transform patterns and good uniformity of displayed bit patterns over 100 bits, have been realized The lowest SNR was 86, and rise time and fall time have been 30 μsec and 8 μsec, respectively, at 190-V halfwave voltage One-dimensional holographic storage experiments have been successfully performed using this PLZT spatial light modulator

Optical Feedback And Bistability: A Review

Abstract: This paper presents a review of the various applications of feedback in optical systems. There are two general areas, optical image processing and bistability. In optical image processing both coherent (optical spatial feedback network internal to an optical resonator) and hybrid (TV) systems are discussed. In the area of bistability there are molecular systems (atoms in an optical resonator), both absorptive and dispersive; hybrid (employing electronic feedback) single-beam and many-element (TV and spatial light modulator-based) systems. All the systems are discussed. A detailed model showing the many facets of molecular bistability is presented along with a discussion of catastrophe theory applied to optical bistability.

Construction of a phase-inverting grating and its use in a deblurring filter.

Abstract: A method of generating phase-inverting grids for use in image deblurring is described. We have also made an amplitude filter for correcting motion blurred images. This was used along with the phase-inverting grid to deblur motion blurred pictures. Experimental results are presented. It is shown that increasing the noise term in a Wiener filter helps in the reduction of noise but occasions loss of resolving power.

Optical apparatus for measuring the size and location of optical in an article

Abstract: In an optical apparatus for inspecting optical defects, a laser beam generated from a laser unit is converted by a collimator lens into a light beam having a suitable diameter. An X-Y scanner for two-dimensionally scanning the light beam is situated at a forward focal point of a scanning lens. The light beam passed through the scanning lens is converged substantially in parallel with an optical axis of the scanning lens and is reflected by a semi-transparent mirror and is vertically projected onto a flat surface to be inspected of an article situated substantially at a backward focal point of the scanning lens. Thus, on the surface to be inspected of the article there is given a projected beam spot having a larger diameter than that of the relevant circle of least confusion. The light rays reflected by the article surface to be inspected are converged by a converging lens the forward focal point of which is located on the said article surface. The light rays regularly reflected by that article surface are converged by the converging lens to a spatial filter and one cut off by the same. Only light rays irregularly reflected by the article surface enters a photo-detecting unit through the spatial filter.